The use of an exhaust gas recirculation (EGR) system to recirculate internal combustion engine exhaust gas to an inlet air path of the engine is well known. EGR can lower the level of certain undesirable engine emission components such as nitrogen oxide (NOx) and can improve fuel economy. Up to a limit, NOx emissions decrease with increasing EGR levels. Beyond the limit, EGR can increase formation of other undesirable engine emission components and can reduce vehicle driveability.
EGR typically involves recirculation of exhaust gas through an EGR passage between an engine exhaust conduit and an engine fresh air intake passage. A valve within the EGR passage (the EGR valve) is controlled to vary a restriction within the EGR passage to regulate the flow of exhaust gas therethrough. When EGR is not required, the EGR valve is driven to a full restriction (closed) position, typically through a spring preload. The spring preload is commonly required to be substantial, to ensure rapid closing of the EGR valve when necessary, and to ensure proper sealing of a closed EGR valve. When EGR is required, the EGR valve is driven to an open position through application of a position control signal to an actuator mechanically linked to the EGR valve. The degree of opening of the EGR valve varies with the magnitude of the position control signal.
The substantial spring preload required for rapid closing and proper sealing of the EGR valve presents a significant EGR control challenge. A large initial control signal, and therefore a large control gain, is required to open the EGR valve from a fully closed position. This large initial control gain can lead to overshoot and transient oscillations within the EGR control system and instability in EGR valve response. In addition to the substantial EGR valve spring preload, constant changes in the engine environment in which the EGR valve is located can present EGR control challenges. For example, engine intake manifold air pressure variation incident on the EGR valve can vary EGR valve response to a position control signal. Temperature within the engine compartment in which the EGR valve and corresponding actuator are located may vary significantly during an engine ignition cycle. Such temperature variations can vary the operating characteristics of the actuator that drives the EGR valve, leading to variation in EGR valve response to a position control signal. System operating voltage variations, for example caused by sudden changes in electrical load, can lead to variation in EGR valve response to a position control signal. Operating voltage may vary from vehicle to vehicle, or within the same vehicle under different operating conditions.
EGR control methods have been proposed. Some of these methods control EGR in accordance with predetermined engine parameters through fixed relationships. Other methods control EGR in accordance with predetermined engine parameters through adjustable relationships, providing for compensation of the EGR control signal in response to changes in engine operating parameters, such as engine speed and load. However, these methods do not compensate for the large initial control signal, and therefore large control gain, required to initially open the valve, thus not providing precision EGR control. Nor do these control methods compensate the EGR control signal for the wide range of operating temperatures and voltages encountered by the EGR valve within the engine environment, thus not providing stable, robust control of the EGR valve.
It would therefore be desirable to reduce the initial position control signal, and therefore the initial control gain, required to initiate movement of the EGR valve from its fully closed position, reducing transient oscillations and overshoot and providing faster and more precise EGR control. It would further be desirable to compensate the initial EGR position control signal in response to variations in the EGR operating environment, for example, manifold air pressure, EGR operating temperature, and system voltage, providing more precise EGR control.